136 PROCEEDINGS OF THE ACADEMY OF 



ai-e much narrower and more embracing. The spire also differs ia being flat 

 instead of convex, as in P. convolutus, and the umbilicus is much deeper, narrower 

 and more defined in the species before us. 



Locality and position. Bed C of the section of freshwater and estuarj deposits 

 at mouth of Judith River. 



Planorbis fragilis. 



Shell rather large, very thin, nearly planiorbicular ; (spire much depressed 

 or flattened ?) ; umbilicus large, deep and exhibiting all the whorls to the apex 

 of the spire; volutions five or six, (much depressed or flat above?) prominent 

 below, rounded on the lower outer side, subangular round the margins of the 

 umbilicus ; (periphery angular ?) ; surface marked by rather faint lines of 

 growth. Greatest diameter 1.23 inches ; height about .40 inch. 



As we have seen oilly imperfect specimens of this shell, we thiijk the above 

 description may have to be modified somewhat when better specimens are 

 obtained. In those we have seen, the volutions appear to have been nearly flat 

 or much depressed above, in consequence of which the outer side of the whorls 

 is distinctly angular. This character, however, and the remarkable flatness of 

 the spire, may be, at least to some extent, caused by pressure. 



Locality and position. — Fort Berthold on the Missouri River, ia Lignite Tertiary 

 basin. 



Melania scbtortuosa. 



Shell conical screw-shaped ; spire not much elevated ; volutions about five, 

 very convex, distinctly angular round the middle, increasing rather rapidly 

 from the apex ; suture strongly defined, in consequence of the prominence of the 

 angular whorls; surface and aperture unknown. Length 29 "inch ; breadth •21 

 inch ; apical angle regular, divergence 47°. 



The angular form of the whorls is so conspicuous a character in this species, 

 that it will be at once known from all its associates, and indeed from all the 

 Nebraska species we have seen, by this character alone. It bears considerable re- 

 Bemblance to M. tortuosa of Anthony, (An. Lye. Nat. Hist., N. Y. vol. 6,) a recent 

 species, but has a shorter spire, and not so many volutions. It is, perhaps, 

 nearer in form to M. acuio-carinata of Lea, but may be distinguished by its more 

 distinctly angular body whorl ; the angle on Mr. Lea's species being only well 

 denned on the spire. 



Locality and position. Mouth Judith River, bed C. of the section of fresh- 

 water and estuary deposits at that place. 



Melania omitta. 



Shell small, very slender ; spire elongate conical, acute at the apex ; volutions 

 about seven, flattened, or very slightly convex, increasing very gradually from 

 the apex; suture linear, not deeply impressed ; surface and aperture unknown. 

 Length -42 inch; breadth -12 ; apical angle regular, divergence 23°. 



Although we have seen neither the aperture nor surface markings of this 

 little shell, we have ventured to characterize it, believing its slender form alone 

 will serve to distinguish it from any of the other species with which it is as- 

 sociated. 



Locality a?id position. Mouth Judith River, in layer of pebbly sandstone of 

 bed G? of the fresh-water and estuary deposits at that place. 



Melania sdbl.'evus. 



Shell elongate conical; spire elevated (acute at the apex?); volutions (about 

 seven?) very depressed convex; suture rather distinctly defined ; surface appa- 

 rently smooth, but when examined with a lens is seen to be marked by fine 

 obscure lines of growth, crossed by extremely fine, nearly obsolete revolving 

 striae; aperture ovate, angular above; columella and outer lip nearly equally 



[May, 



